(1) Field of the Invention
The present invention relates to a diversity transmission-reception system, and more particularly to a time diversity transmission-reception system utilizing code division multiplexing.
(2) Description of the Related Art
Diversity reception is generally necessary for radio communication involving fading channels, and particularly for digital communication through fading multipath channels. Signal fading is classified into two, i.e., flat fading and frequency-selective fading. Flat fading occurs as the amplitude and phase variation in the propagated signal, which has been received directly rather than through fading multipath channels. In contrast, frequency-selective fading occurs as a result of propagation through fading multipath channels, each of which causes mutually independent amplitude and phase variations to the signal propagated therethrough. In frequency-selective fading, where the reception signal is obtained through combining a plurality of signals propagated through fading multipath channels, the received signals at some frequencies may have mutually opposite phases resulting in zero amplitude. This causes frequency-selective fades or notches in the frequency spectrum of the reception signal. While the effect of flat fading is limited to the level of the reception signal with the waveform thereof unaffected, the frequency-selective fading resulting from propagation through fading multipath channels causes the variation not only in reception signal level but also in its waveform.
To obviate the adverse effect of fading multipath channels, diversity techniques and adaptive equalization techniques have been conventionally utilized. Among a variety of possible combinations of these techniques, the present invention is directed to the use of time diversity combined with adaptive equalization.
Referring to FIGS. 1(a) and 1(b) showing respectively in blocks the transmit and receive sides of a conventional time diversity transmission-reception system, a data signal (data symbol sequence) "a" to be transmitted is split into two and supplied, one through delay means 101 (having delay t) and the other directly, to a pair of modulators 102 for modulation into modulated intermediate frequency (IF) signals, and then to a pair of transmitters 103 of carrier wave radio frequencies (RF) f.sub.1 and f.sub.2, whose outputs are combined at a combiner 104 and transmitted through a transmitting antenna 105. As a result, the two-branch transmit signals are transmitted at radio frequencies f.sub.1 and f.sub.2 with a time spacing of t. On the receive side shown in FIG. 1(b), the RF signal received by receiving antenna 106 is applied to a branching filter 107, which splits the received RF signal into the f.sub.1 and f.sub.2 components. These frequency components are respectively supplied to a pair of receivers 108 for amplification and conversion into intermediate frequency (IF) and then to a pair of demodulators 109 for synchronous detection. The output of demodulator 109 derived from the f.sub.1 component is then supplied through delay means 110 of delay t to optimum signal selector/signal combiner 111, while the output of demodulator 109 derived from the f.sub.2 component is supplied directly to selector/combiner 111. The selector/combiner 111 may consist of a signal selector for selecting the demodulation output signal of better quality out of the two incoming demodulation output signals in response to bit error rates and/or loss of frame synchronization. Alternatively, selector/combiner 111 may be composed of a diversity signal combiner designed to combine the two incoming demodulation output signals after phase-controlling them into timed state. The signal combiner may be made of a maximal ratio combiner designed not only to control the phases of the incoming signals into timed state but also to control their amplitudes to provide squared and unsquared values thereof. The output of selector/combiner 111 is supplied to adaptive equalizer 112 for eliminating waveform distortions caused by propagation through fading multipath channels, to provide reception data signal A.
The conventional system shown in FIGS. 1(a) and 1(b) may be called a frequency diversity system because two carrier waves of radio frequencies f.sub.1 and f.sub.2 are employed. However, this conventional system cannot achieve the frequency diversity effect if the f.sub.1 -f.sub.2 correlation is high enough to make the separation therebetween inadequate. On the other hand, delay t given to the second branch transmit signal provides the time diversity effect, if delay t is set at a value longer than the fading period.
In general, the f.sub.1 -f.sub.2 correlation cannot be reduced without expanding the frequency separation therebetween, which is undesirable from the viewpoint of the more efficient use of frequencies. Therefore, the conventional system shown in FIGS. 1(a) and 1(b) depends more heavily on time diversity, with the frequency diversity used only for the purpose of providing time diversity branches. However, the use of two radio frequency carrier waves at frequencies f.sub.1 and f.sub.2 requires the use of two transmitters 103 and two receivers 108, which are generally of large scale and costly to manufacture. Furthermore, the problem of dimensions, scale and manufacturing cost of the transmitter/receiver for the conventional system becomes more serious when the number of diversity branches is increased to more than two.
Another prior art diversity transmission-reception system, which is outlined in Japanese Patent Application Kokai Publication No. Sho 63-286027 entitled "Transmission-path diversity-transmission system" and published Nov. 22, 1988, has, as schematically shown in FIG. 2, modulator 201 for data signal, whose output is supplied to a first transmitting antenna 203 directly and also to a second transmitting antenna 204 through a delay circuit 202 having delay t. On the receive side, the reception system has one receiving antenna 205, whose output is supplied to receiver 206, where the received RF signal is converted into an IF signal for detection by detector 207. The output of detector 207 is applied to waveform equalizer 208 for code decision by decision circuit 209. The delay t introduced by delay circuit 202 is set at a value longer than one time slot assigned to each of the modulating data symbols. The transmission carrier wave transmitted from the first and second transmitting antennas is propagated through mutually independent propagation paths, and subsequently received by a single receiving antenna 205. Therefore, the propagated signal consists of a plurality of multipath waves, whose undelayed and delayed components have respectively undergone Rayleigh fading mutually independently. Waveform equalizer 208 is designed to select either the undelayed or delayed wave, while eliminating the unselected wave, thereby to realize two-branch selective diversity reception. It is also described in the above-mentioned Kokai Publication that the undelayed and delayed waves are time-adjusted and combined by maximal ratio combining.
The prior art system of FIG. 2 is based on the concurrent use of time diversity and space diversity reception systems in that it utilizes the absence of spatial correlation among propagation paths. While a space diversity reception system ordinarily requires a plurality of receiving antennas, the above-cited prior art system has two transmitting antennas and a single receiving antenna, reducing the equipment size on the receive side and curtailing the manufacturing cost. The reduction of the number of receiving antenna to one is significant for a microwave communication system, which requires large-aperture antennas and associated equipment in contrast to terrestrial mobile communication systems requiring only small-sized antennas.
In the conventional diversity reception systems outlined above, either frequency or space (propagation path) must be relied on as transmission media. Particularly, in the time diversity reception concurrently employing frequency diversity for separation and derivation of diversity branches, the expansion of required frequency bandwidth and equipment scale is unavoidable, resulting in increased manufacturing cost. Similarly, the time diversity reception accompanied by space (propagation path) diversity requires a plurality of antennas, increasing the manufacturing cost particularly when the antennas are of large aperture type. Furthermore, the increase in the number of diversity branches in the above-mentioned time diversity reception system accompanied by either frequency or space diversity makes the abovementioned problems more serious.